More specifically, it concerns the separation of the isomers of crown compounds with the formula: ##STR1## in which n=0 or is a whole number ranging from 1 to 3, R.sub.1 and R.sub.2, which are identical or different, are alkyl or alkoxyalkyl radicals or R.sub.1 and R.sub.2 collectively form a cycloalkyl radical.
From these crown compounds, dicyclohexyl 18-Crown-6 (DCH18C6) which responds to the formula: ##STR2## is a substance commercially available in the form of a mixture of several isomers.
In fact, this product has 5 diastereoisomers having the following structures: ##STR3##
When the DCH18C6 is obtained via the catalytic hydrogenation of dibenzo-18-Crown-6, as is the case with commercial products, it mainly contains cis-syn-cis (isomer A) and cis-anti-cis (isomer B) isomers.
For certain applications, it may be advantageous to use one of the pure isomers rather than the mixture of isomers. However, all the known methods to date to separate these various isomers with satisfactory yields are difficult to implement on an industrial scale.
One of these known methods is a method to separate the isomers from DCH18C6 via chromatography on an aluminium column, this method using a hexane/ether mixture whose ether concentration gradient gradually increases so as to elute the isomer A, and methanol to collect the isomer B. This method described by R. M. Izatt and al in J. Amer. Chem. Soc., 93, 1619 (1971) has the drawback of being long and expensive to implement and only provides yields of 20 to 30% for each of the isomers.
R. N. Izatt and al have described in Inorganic Chemistry, 14, 3132 (1975) another method to separate the isomers from DCH18C6 by a method based on the selective precipitation of the lead DCH18C6/perchlorate complex of the isomer B (cis-anti-cis).
After separation of the precipitate including the isomer B by filtering and processing the filtrate with H.sub.2 S to eliminate the lead, the isomer A (cis-syn-cis) remaining in the solution is recovered by adding perchloric acid so as to precipitate the isomer A. Although this method results in obtaining high yields, it may not be used easily on an industrial scale, as it requires the use of the heavy metal perchlorates known to be explosive and sulphurated hydrogen known to be toxic.
Another method for separating isomers from DCH18C6 described by C. J. Pedersen in Organic Syntheses, 52, 66, uses crystallization of the potassium acetate/DCH18C6 complex in a petroleum ether/CH.sub.2 Cl.sub.2 mixture.
This method has the drawback of only being suitable for separation of the isomer B (cis-anti-cis) and only results in obtaining a yield of 12%.
Therefore, the known methods for separating the isomers from DCH18C6 are either less high-performing, expensive and long, or are impossible to implement on an industrial scale.